In an MRI apparatus (or an MR scanner), an examination object, usually a patient, is exposed within the examination space of the MRI apparatus to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object tend to rotate around the axis of the applied B0 field (Larmor precession) with a certain net magnetization of all nuclei parallel to the B0 field. The rate of precession is called Larmor frequency which is dependent on the specific physical characteristics of the involved nuclei and the strength of the applied B0 field.
By transmitting an RF excitation pulse (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna or coil, and matching the Larmor frequency of the nuclei of interest, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation pulse, the MR relaxation processes of the longitudinal and transversal components of the net magnetization begin, until the net magnetization has returned to its equilibrium state. MR relaxation signals which are emitted by the relaxation processes, are detected by means of an RF/MR receive antenna or coil. The received MR signals which are time-based amplitude signals, are Fourier transformed to frequency-based MR spectrum signals and processed for generating an MR image of the nuclei of interest within an examination object. In order to obtain a spatial selection of a slice or volume of interest within the examination object and a spatial encoding of the received MR relaxation signals emanating from a slice or volume of interest, gradient magnetic fields are superimposed on the B0 field, having the same direction as the B0 field, but having gradients in the orthogonal x-, y- and z-directions. Due to the fact that the Larmor frequency is dependent on the strength of the magnetic field which is imposed on the nuclei, the Larmor frequency of the nuclei accordingly decreases along and with the decreasing gradient (and vice versa) of the total, superimposed B0 field, so that by appropriately tuning the frequency of the transmitted RF excitation pulse (and by accordingly tuning the resonance frequency of the RF/MR receive antenna), and by accordingly controlling the gradient magnetic fields, a selection of nuclei within a slice at a certain location along each gradient in the x-, y- and z-direction, and by this, in total, within a certain voxel of the object can be obtained.
The above RF/MR (transmit and/or receive) antennas can be provided both in the form of so-called body coils (also called whole body coils) which are fixedly mounted within an examination space of an MRI system for imaging a whole examination object, and as so-called surface or local coils which are arranged directly on or around a local zone or area to be examined and which are constructed e.g. in the form of flexible pads or sleeves or cages like head coils. Both the above types can be realized in the form of a volume resonator (like a birdcage coil or a TEM resonator) or in the form of a (planar or volume) array antenna (or array coil) which comprises a set of coil elements which are decoupled from one another and which each transmit/receive their own localized magnetic field.
For MR imaging an examination object like a patient, the nuclei of interest in human tissue are usually 1H protons, so that the above RF transmit/receive antennas are accordingly tuned to the Larmor frequency of 1H protons. However, for imaging certain organs by means of contrast agents, for detecting and quantifying labeled tracers and drugs in the field of molecular imaging, for examining metabolism or for MR spectroscopy, other nuclei are of interest to be imaged like especially 19F, 3He, 13C, 23Na and other, wherein all these nuclei have substantial different Larmor frequencies. Consequently, for MR image generation, the above RF/MR transmit/receive antennas have to be tuned to two or more different Larmor frequencies in order to image both the structure of the examination object and the desired other nuclei in a common overlay of one another.
U.S. Pat. No. 7,508,212 discloses an RF resonator having a cylindrical shield formed around a central axis and a plurality of pairs of opposing conductive coil legs arranged symmetrically around the central axis, wherein the pairs of opposing conductive legs being divided into a first set and a second set, wherein the first set is tuned to a first Larmor frequency and the second set is tuned to a second Larmor frequency. Further, a first drive circuitry is connected to each pair of opposing legs in the first set and a second drive circuitry is connected to each pair of opposing legs in the second set, wherein each drive circuitry comprises current baluns for electrically isolating the drive circuitry from the coil legs, and tune and match circuits for conducting multi-nuclear measurements simultaneously at different Larmor frequencies.
U.S. Pat. No. 5,168,230 discloses a dual frequency NMR coil pair which is comprised of two individual coils tuned to separate resonant frequencies. Each coil is formed into approximately the same shape by a conductive loop which follows a serpentine path to define an inner area and a plurality of outer lobes. The two individual coils are positioned in close proximity overlying each other, but rotated with respect to each other such that the outer lobes of the two respective coils are interleaved. By this, a mutual loading between the two individual coils shall essentially be eliminated, permitting a dual frequency operation with minimal degradation of signal to noise ratio in either coil, while both coils substantially have the same field of view.
US 2009/0160442 discloses an MR transmit/receive coil which can resonate at least at two different (Larmor) frequencies. It comprises a tuning resonant circuit which is serially coupled into the coil and which comprises tuning components, the values of which are selected such that a sensitivity profile of the coil resonating at the first resonant frequency substantially matches the sensitivity profile of the coil resonating at the second resonant frequency.
The international application WO2010/018535 mentions that an RF TEM coil built from strip lines can be made multi-resonant. Further, the TEM strip lines are connected via impedance networks to a transmitter or receiver.